1. Field of the Invention
This invention relates to an improvement in processes for cleaning totally or partially dried residues of paints and like materials from surfaces, particularly surfaces that are not readily accessible to access by conventional cleaning tools such as scrapers, abrasive bodies, and the like. More particularly, this invention relates to cleaning of pipework and similar structures that convey liquid paints and like materials to apparatus by which the paint is applied to form a protective and decorative coating on manufactured objects such as automobile bodies and appliance cabinets. Normally in such manufacturing operations, at least the color and sometimes more fundamental characteristics of the protective and decorative coatings applied from the same apparatus are varied from time to time.
Whenever such a change is made, it is necessary to clean the liquid paint conveying structures in order to prevent inconsistent product appearances resulting from contamination of one type of paint by another. Cleaning would be relatively easy if it could be assured that any unused protective coating forming liquid product would remain liquid in all parts of the conveying apparatus throughout all uses of the apparatus, including changing from one color or type of protective coating to another. However, as is well known to those skilled in the art, such a condition rarely if ever is observed in practice, and it is therefore necessary to have a process of cleaning that can satisfactorily clean surfaces on which protective coating forming liquids have been partially or totally dried. This general field will be denoted briefly hereinafter as "paint line cleaning", but it is to be understood that the invention applies to cleaning any type of surface on which an unwanted and at least partially solid coating has formed as a result of contact with a liquid composition intended to form a protective solid coating on other surfaces.
2. Discussion of Related Art
Traditionally, paint line cleaning has been accomplished primarily by contacting an at least partially solid coating to be removed with a liquid solvent or swelling agent for the solid protective coating. Such a process is fully satisfactory only in the relatively few cases in which the protective coating is readily soluble in some reasonably priced solvent. Some highly effective and frequently used protective coatings, however, are chemically cross-linked to varying degrees, and usually no true solvent at all is known for densely cross-linked solids. Organic solids of the types used for most paints or like protective coatings, whether crosslinked or not and even if insoluble in any known liquid, can usually be swollen by some liquids, despite their lack of true solubility, and the swollen solids are mechanically weaker and therefore more readily removable by mechanical action. However, obtaining effective mechanical action in the interior of a long narrow pipe remains a difficult problem. One previously attempted solution is to incorporate conventional abrasive particles into a slurry in a suitable swellant/solvent for the dried paint residues to be removed and then pump this slurry through the pipe to be cleaned. This method does improve the removal of paint but introduces a new problem: Conventional abrasive particles themselves are difficult to remove completely from a paint line, and they are extremely undesirable contaminants in liquid coating forming compositions intended to produce most decorative surfaces, because they often cause readily visible surface irregularities even when relatively small.